1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer and a copying machine, for forming an image on a recording medium.
2. Description of the Related Art
Hitherto, an electrophotographic image forming apparatus is known in which an electrostatic latent image is formed on an electrophotographic photoconductor (referred to simply as a “photoconductor” hereinafter), serving as an image carrier, with exposure of, e.g., a laser beam emitted in response to an image signal, the electrostatic latent image is visualized into a developer (toner) image using a developer, and a hard image is obtained by transferring the toner image onto a transfer material and then fusing it.
Also, an electrophotographic image forming apparatus for forming a color image is known in which developer (toner) images of multiple colors formed on a photoconductor are successively transferred onto a recording medium (also referred to as a “transfer material” hereinafter), or in which developer (toner) images of multiple colors formed on a photoconductor are primary-transferred onto an intermediate transfer member and then secondary-transferred onto a transfer material, whereby a color toner image is formed on the transfer material.
In an electrophotographic image forming apparatus, if variations occur in performance of apparatus components with changes of environment around the apparatus and use of the apparatus for a long period, the density of a toner image formed on a transfer material by the image forming apparatus also varies. In an electrophotographic image forming apparatus for forming a color image, particularly, there is a risk that a color balance is lost even with a slight variation in density of the toner image. It is hence desired to keep constant the image density and gradation characteristics at all times regardless of variations in performance of the apparatus components.
In an image forming apparatus for forming a color image, therefore, a method for keeping constant the image density and gradation characteristics (i.e., color balance) at all times is proposed and comprises, for example, the step of changing process conditions such as an amount of laser exposure and a development bias, or adjusting the compensation factors set in a lookup table (LUT) which is used to modify an image signal for forming an electrostatic latent image on a photoconductor with the laser exposure, depending on changes of environment (e.g., absolute temperature) around the apparatus and variations in performance of the apparatus components.
Also, as a method for ensuring constant the image density and gradation characteristics in spite of variations in performance of the components of the image forming apparatus, it is conceivable to form a pattern of a reference developer image (referred to as a “toner patch” hereinafter) for density detection on a photoconductor or an intermediate transfer member, and to detect the density of the toner patch with a photosensor. That method enables the image density and gradation characteristics (i.e., color balance) to be kept constant at all times by changing process conditions such as an amount of laser exposure and a development bias, or by modifying an image signal based on a lookup table (LUT) which is used to modify the image signal for forming an electrostatic latent image on a photoconductor with the laser exposure, in accordance with a result detected by a photosensor.
In the above-mentioned toner image density control using a photosensor, however, the density is detected using a toner patch formed on the photoconductor or the intermediate transfer member, and the control is not intended to compensate for a change in image color balance caused by transferring and fusing a toner image onto a transfer material. Further, it is known that the image color balance is also changed depending on not only the efficiency in transfer of a toner image onto a transfer material, but also heat and pressure applied during the fusing.